Why did South Dakota vote the way it did from 1896 to 1916?

[Kingpoleon]

It voted Bryan 1896/Roosevelt 1904/Roosevelt 1912/Hughes 1916. What caused the big transition from 1896 to the 1920s? Was it Democrats failing to keep the Populists strong in their party? Charles W. Bryan was the last person from the Plains to be on a Democratic presidential ticket which won more than one state.

[TDAS04]

1896: It voted for Bryan (barely) since there was a strong Populist Party here in the 1890s, as there was in other Plains States.  Maybe it was because of German skepticism of Bryan that the margin wasn't larger (ND & MN went handily for McKinley). 

1904: TR was popular in the entire region, including Populist strongholds.

1912: TR was helped by the absence of Taft on the ballot.  Maybe Wilson would have won South Dakota if Taft had been there to split the Republican vote.

1916: It was close.  (Interestingly, Hughes strength was east of the Missouri River, and Wilson carried western SD.  A reverse of today, where West River is super Republican and East River is more moderate.  Of course, West River is more like MT & WY, which went handily for Wilson.)

South Dakota showed early GOP strength because the populated eastern portion is similar to MN & IA, which were Republican states early on.  It stayed Republican as the less Scandinavian and more individualistic West River gained influence, which is a reason why South Dakota never had the strong progressive, communitarian streak of Minnesota or North Dakota (which had the Farmer-Labor Party and the Non-Partisan League, respectively).